Controlled warm-rolling method

ABSTRACT

The invention presents a new warm control rolling method, in consideration of processing heat generation, as a method of stably manufacturing ultrafine crystal steel of 3 microns to 1 micron or less, without any limitation in pass interval or strain speed, being a rolling method of manufacturing steel mainly composed of fine ferrite particle texture with average ferrite grain size of 3 μm or less, in which, in the rolling process of one pass or more wherein the rolling temperature range is a temperature region of 350° C. to 800° C., the material temperature upon start of rolling of each rolling process does not exceed the maximum temperature of 800° C., and the material temperature during rolling and right after final rolling (within 1 second) is not lower than 350° C., temperature T x-out  right after rolling in each rolling process (within 1 second) is not higher than the temperature that is higher than rolling entry temperature T x-in  by 100° C. and the material temperature right after rolling (within 1 second) is not lower than the temperature that is lower than the temperature right before rolling by 100° C.

TECHNICAL FIELD

The present invention relates to a warm control rolling method. Morespecifically, the invention relates to a manufacturing method ofultrafine particle steel material excellent in strength and ductilityhaving ultrafine crystal structure of grain size of 3 μm or less.

BACKGROUND ART

Ultrafine particle steel can extremely enhance the strength withoutadding alloy elements, and simultaneously reduces the ductility andbrittleness transition temperature extremely, and hence the presentinventors have been studying about measures for realizing this ultrafineparticle steel industrially, and have already disclosed inventions, warmmultipass rolling method (document 1) and multidirectional processingmethod (document 2).

On the other hand, various methods have been proposed so far formanufacturing ultrafine particle steel, but nothing has been known aboutthe method of controlling the grain size quantitatively.

For example, Fujioka et al. (document 3) proposed a manufacturing methodof high tension steel characterized in using a billet comprising C, 0.03to 0.45 weight (wt.) %, Si: 0.01 to 0.50%, Mn: 0.02 to 5.0%, Al: 0.001to 0.1%, and balance of Fe and inevitable impurities, processing it byone pass or two or more consecutive passes with interval of each pass ofwithin 20 seconds, at temperature of 500 to 700° C., strain speed of 0.1to 20/sec, and total strain amount of 0.8 to 5.0, and then coolingslowly.

They also proposed a manufacturing method of high tension steel of finecrystal particles characterized in using a billet comprising, by wt. %,C, 0.03 to 0.9, Si: 0.01 to 1.0, Mn: 0.01 to 5.0, Al: 0.001 to 0.5, N,0.001 to 0.1, and also at least one of Nb: 0.003 to 0.5 and Ti: 0.003 to0.5, and balance of Fe and inevitable impurities, and satisfying therelation of C+(12/14)N≧(12+48)Ti(12/487)Nb+0.03, casting it or heatingit and cooling it once into the temperature range of 500° C. to roomtemperature after rolling or without rolling, heating,

rolling in warm process at 700 to 550° C. by processing it in one passor two or more consecutive passes with pass interval of within 10seconds with draft per pass of 20% or more, at strain speed of 1 to 200sec, and total strain amount of 0.8 to 5, and then cooling slowly(document 4).

In these two methods, however, nothing is taught about control method ofgrain size. In these methods, moreover, the pass duration is limited andthe strain speed is also limited, and it is considered difficult toapply industrially.

In this background, the inventors have further promoted studies, andfound that it is important to control the cumulative strain in multipassrolling, processing temperature, strain speed, and pass durationcomprehensively, not individually, in order to form an ultrafine crystalstructure. As a result, it is known that the grain size depends onparameter Z of processing temperature and strain speed expressed informula (1); and the present inventors have proposed a new controlmethod of grain size by clarifying the relation of Z and grain sizethrough one-pass rolling experiment (prior application 1).

$\begin{matrix}{Z = {\log\left\lbrack {\frac{ɛ}{t}{\exp\left( \frac{Q}{8.31\left( {T + 273} \right)} \right)}} \right\rbrack}} & (1)\end{matrix}$

-   -   ε: strain    -   t: duration from start till end of rolling (s)    -   T: rolling temperature (° C., or average of rolling temperature        of each pass in the case of multipass rolling)    -   Q: 254,000 if mother phase of texture just before rolling is        ferrite, bainite, martensite, or pearlite; 300,000 if mother        phase is austenite.

In this method, to manufacture ultrafine ferrite steel with crystal sizeof 1 microns or less, it is found that the rolling condition parameter Zin formula (1) should be 11 or more (in the case the texture beforerolling is ferrite, bainite, martensite, pearlite or the like, that is,the iron crystal structure is bcc), and that the strain speed can bedefined by the value of the total strain ε being divided by the time tfrom start till end of rolling, and hence ultrafine crystal particles of1 micron or less can be obtained in the condition of strain e=3.0 andtotal rolling time t=300 s, that is, strain speed=0.01/s, and this newlyproposed method can be applied in a wide range.

According to this method, the grain size can be controlled withoutlimitation in pass interval or strain speed.

However, in the subsequent process of researches by the presentinventors, new problems have been also unveiled, that is, the actualrolling is continuous multipass process, and when the parameter Z is 11or more, the rolling temperature corresponds to the warm processingtemperature region (350 to 800° C.), and the deformation resistance ofsteel is large and the processing heat generation of material is largein this case, and the material temperature may rise several hundreddegrees during continuous rolling, there by resulting in Z<11, andultrafine structure of 1 micron may not be formed.

It has been hence demanded to develop a method capable of controllingthe grain size stably even in such continuous multipass rolling.

Document 1: Japanese Patent Application Laid-Open No. 2000-309850

Document 2: Japanese Patent Application Laid-Open No. 2001-240912

Document 3: Japanese Patent Application Laid-Open No. 9-279233

Document 4: Japanese Patent Application Laid-Open No. 2000-104115

Prior application 1: Japanese Patent Application No. 2002-54670

The present invention is applied in the light of the above background,and presents a new control rolling method in consideration of processingheat generation, as a method capable of applying the new method ofcontrolling the parameter Z according to the proposal of the inventorsin continuous rolling process, stably manufacturing ultrafine crystalsteel of 3 microns to 1 micron or less without any limitation in passinternal or strain speed.

DISCLOSURE OF INVENTION

To solve the problems, it is a first aspect of the invention to presenta rolling method of manufacturing steel mainly composed of fine ferriteparticle texture with average ferrite grain size of 3 μm or less, whichcomprises, in the rolling process of one pass or more of rolling incondition where the rolling condition parameter expressed in formula (1)

$\begin{matrix}{Z = {\log\left\lbrack {\frac{ɛ}{t}{\exp\left( \frac{Q}{8.31\left( {T + 273} \right)} \right)}} \right\rbrack}} & (1)\end{matrix}$

-   -   ε: strain    -   t: duration from start till end of rolling (s)    -   T: rolling temperature (° C., or average of rolling temperature        of each pass in the case of multipass rolling)    -   Q: 254,000 if mother phase of texture just before rolling is        ferrite, bainite, martensite, or pearlite; 300,000 if mother        phase is austenite.        is 11 or more (in the case the texture just before rolling is        ferrite, bainite, martensite, or pearlite, that is, Fe crystal        structure is bcc) or 20 or more (in the case the texture just        before rolling is austenite, that is, Fe crystal structure is        fcc), and the rolling temperature range is a temperature zone of        350° C. to 800° C., rolling under condition that the material        temperature upon start of rolling of each rolling process does        not exceed the maximum temperature of 800° C. and the material        temperature during rolling and right after final rolling (within        1 second) is not 350° C. or lower, and rolling so that, in each        rolling process, temperature T_(x-out) right after rolling        (within 1 second) is not higher than temperature that is higher        than rolling entry temperature T_(x-in) by 100° C. and the        material temperature right after rolling (within 1 second) is        not lower than temperature that is lower than the temperature        right before rolling by 100° C.

Herein, the strain used in formula (1) is an industrially simple strain,that is, true strain e. For example, supposing the initial area of steelbar to be S₀ and the C-section area after rolling to be S, the reductionof area R is expressed asR=(S ₀ −S)/S ₀  (2)Hence, the true strain e ise=−Ln(1−R)Instead of the true strain, plastic strain obtained by finite elementmethod may be used. Calculation of plastic strain is specificallyexplained in reference documents (Inoue, et al., “Iron and Steel”, 86(2000) 793; Keizaburo Harumi, et al., “Introduction to finite elementmethod” (Kyoritsu Publishing), Mar. 15, 1990).

The rolling time t may be the total rolling time including the passintervals.

It is a second aspect to present the warm control rolling method, beingcharacterized in rolling so that the temperature T_(x-out) right afterrolling in each rolling process is not higher than the temperature thatis higher than the rolling entry temperature T_(x-in) by 50° C.

It is a third aspect of the invention to present the warm controlrolling method, being characterized in rolling two or more passesconsecutively in rolling temperature range of 350° C. to 800° C., inwhich the material temperature right after two passes is not higher thanthe temperature that is higher than the material temperature upon startof rolling by 100° C., and not lower than the temperature that is lowerthan the material temperature upon start of rolling by 100° C., and itis a fourth aspect to present the warm control rolling method, beingcharacterized in rolling so that the material temperature right aftertwo passes is not higher than the temperature that is higher than thematerial temperature upon start of rolling by 50° C.

It is a fifth aspect of the invention to present the warm controlrolling method, being characterized in rolling in rolling temperaturerange of 400° C. to 500° C., it is a sixth aspect to present the warmcontrol rolling method, being characterized in manufacturing steel withZ≧12 or more and mainly composed of texture with average ferrite grainsize of 1 μm or less, it is a seventh aspect to present the warm controlrolling method, being characterized in starting rolling, in consecutivemultipass rolling, by waiting until the rolling entry temperatureT_(x+1-in) of X+1-th pass becomes T_(S)+20≧T_(x+1-in) when the rollingtemperature T_(x-out) right after X-th pass is higher than the rollingset temperature T_(S), and it is an eighth aspect to present the warmcontrol rolling method, being characterized in measuring the processingheat generation T_(xH) at X-th pass in multipass rolling beforehand, anddefining the rolling entry temperature T_(x-in) in the relation ofT_(xs)≧T_(x-in)≧T_(xS)−T_(xH), supposing T_(xS) to be rolling settemperature.

It is a ninth aspect of the invention to present the warm controlrolling method, being characterized in defining the total reduction areaat 50% or more in continuous rolling, it is a tenth aspect to presentthe warm control rolling method, being characterized in defining theplastic strain, or the strain converted into true strain from thereduction of area at 1.5 or more, it is an eleventh aspect to presentthe warm control rolling method, being characterized in introducing thestrain by multidirectional processing, it is a twelfth aspect to presentthe warm control rolling method, being characterized in controlling thetemperature range before and after rolling by setting the rolling speedand the draft of each pass, and it is a thirteenth aspect to present thewarm control rolling method, being characterized in a step of reheatingin the midst of rolling for compensating for temperature drop ofmaterial, and a step of cooling in the midst of rolling for suppressingtemperature rise of material in the continuous rolling.

In the invention of the present application, having such configuration,the new method of controlling by the parameter Z relating to theproposals of the inventors can be applied in the continuous rollingprocess, and a new control rolling method in consideration of processingheat generation can be realized as a method of stably manufacturingsuperfine crystal steel of 3 microns to 1 micron or less without anylimitation in the pass interval or strain speed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing the relation of parameter Z and ferriteaverage grain size relating to formula (1).

FIG. 2 is front view and dimensional drawing showing caliber shape ofgroove roll of each pass.

FIG. 3 is a texture SEM image in embodiment 1.

FIG. 4 is a texture SEM image in embodiment 2.

FIG. 5 is a texture SEM image in embodiment 3.

FIG. 6 is a texture SEM image in embodiment 4.

FIG. 7 is a texture SEM image in comparative example.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention has many features as discussed above, and its embodimentsare specifically described below.

In the method of the invention, in order to manufacture fine ferriteparticle structure with average ferrite grain size of 3 μm or less asmain constituent, that is, to manufacture steel having ultrafine textureof average ferrite grain size of 3 μm or less in 60% or more range ofsurface area of its section, in principle, the rolling is executed inthe following condition:

<A> In a range of rolling condition parameter Z expressed in formula (1)is 11 or more (in the case the texture before rolling is ferrite,bainite, martensite, pearlite or the like, that is, iron crystalstructure is bcc) or 20 or more (in the case the texture before rollingis austenite, that is, iron crystal structure is fcc), and

<B> In the rolling process of one pass or more with rolling temperaturerange of a temperature region of 350° C. to 800° C., the materialtemperature upon start of rolling of each rolling process does notexceed the maximum temperature of 800° C., and the material temperatureduring rolling and right after final rolling (within 1 second) is notlower than 350° C., and then temperature T_(x-out) right after rollingin each rolling process (within 1 second) is not higher than thetemperature that is higher than rolling entry temperature T_(x-in) by100° C., and the material temperature right after rolling is not lowerthan a temperature that is lower than the temperature right beforerolling by 100° C.

According to the studies by the inventors, the parameter Z in formula(1) is known to be an easy index for obtaining ultrafine crystalstructure of average grain size, and has been already proposed in theapplication of Japanese Patent Application No. 2002-54670. The inventorshave studied and clarified that the average grain size of ultrafinecrystals formed by warm processing depends on the processing temperatureand strain speed, and that the crystal grain size becomes smaller alongwith increase of rolling condition parameter Z which is a function ofprocessing temperature and strain speed. To obtain texture of averagegrain size of 1 μm or less, the rolling condition parameter must be sethigher than a certain critical value. As a result of experiment byone-pass large strain compression process using small sample pieces, thecritical value is confirmed to be about 11 in the case of bcc structureiron (ferrite, bainite, martensite, pearlite or the like), and about 20in fcc structure iron (austenite) (FIG. 1).

Herein, the strain used in formula (1) may be an industrially simplestrain, that is, true strain e. For example, supposing the initial areaof steel bar to be S₀ and the C-section area after rolling to be S, thereduction of area R is expressed asR=(S ₀ −S)/S ₀  (2)Hence, the true strain e ise=−Ln(1−R)Instead of the true strain, plastic strain obtained by finite elementmethod may be used. Calculation of plastic strain is specificallyexplained in reference documents (Inoue, et al., “Iron and Steel”, 86(2000) 793; Keizaburo Harumi, et al., “Introduction to finite elementmethod” (Kyoritsu Publishing), Mar. 15, 1990).

More specifically, the plastic strain can be calculated according to theflow shown in Table 1.

TABLE 1 Calculation flow of plastic strain 1. Acquire stress-straincurve corresponding to the processing temperature of material. 2.Prepare for finite element method calculation. (1) Form mesh on theworkpiece. (2) Determine contact condition; coefficient of friction =0.3 coulomb condition. (3) Determine stress-strain curve and materialphysical properties. 3. In the conditions of (1) to (3), calculate bythe all- purpose finite element method code, for example, ABAQUS.Plastic strain ε is expressed in the following formula, and each strainincrement is calculated by the all-purpose finite element method code.$\begin{matrix}{ɛ = {\frac{2}{3}\sqrt{\begin{matrix}\left\lbrack {{\frac{1}{2}\left\{ {\left( {{d\; ɛ_{x}} - {d\; ɛ_{y}}} \right)^{2} + \left( {{d\; ɛ_{y}} - {d\; ɛ_{z}}} \right) + \left( {{d\; ɛ_{z}} - {d\; ɛ_{x}}} \right)^{2}} \right\}} +} \right. \\\left. {\frac{3}{4}\left( {{d\;\gamma_{xy}^{2}} + {d\;\gamma_{yz}^{2}} + {d\;\gamma_{zx}^{2}}} \right)} \right\rbrack\end{matrix}\;}}} \\{{d\; ɛ_{x}},{d\; ɛ_{y}},{d\; ɛ_{z}\text{:}\mspace{14mu}{strain}\mspace{14mu}{increment}\mspace{14mu}{of}\mspace{11mu} x},y,z} \\{{d\;\gamma_{xy}},{d\;\gamma_{yz}},{d\;\gamma_{zx}\text{:}\mspace{14mu}{strain}\mspace{14mu}{increment}\mspace{14mu}{of}\mspace{11mu} x},y,z}\end{matrix}\quad$

In the warm control rolling method of the invention, accordingly, therolling condition is set so that the parameter Z may be 11 or more (bccstructure) or 20 or more (fcc structure) as specified in <A>.

More important, the temperature is controlled characteristically as in<B> in the warm control rolling method of the invention.

Microscopic local azimuth differences caused by machining large strainsby warm processing become sources of ultrafine crystals, and in therecovery process taking place during processing or after processing,dislocation density in particle declines, and crystal grain boundary isformed at the same time and ultrafine texture is composed. If thetemperature is low, recovery is insufficient, and processed texture ofhigh dislocation density is left over. If the temperature is too high,discontinuous recrystals or crystal grains by ordinary particle growthbecome coarse, and ultrafine particle texture of 3 μm or less is notobtained. In the invention, therefore, the rolling temperature islimited in a range of 350° C. to 800° C.

Rolling is executed by controlling as follows: temperature T_(x-out)right after rolling (within 1 second) in each rolling is not higher than100° C. than rolling entry temperature T_(x-in), and the materialtemperature right after rolling is not lower than 100° C. than thetemperature before rolling.

This temperature control is also essential for the invention. Withoutsuch control, actually, if the parameter Z is within the specifiedrange, it is difficult to control within the specified crystal grainsize at grain size of 3 μm or less.

T_(x-out) is not higher than a temperature that is higher than T_(x-in)by 100° C., and more preferably by 50° C. In consecutive rolling of twoor more passes, preferably, the material temperature right after twopasses should not be higher than a temperature that is higher than thematerial temperature upon start of rolling by 100° C., more preferablyby 50° C., and should not be lower than a temperature that is lower thanthe material temperature upon start of rolling by 100° C.

To obtain texture of ultrafine particles with average ferrite grain sizeof 1 μm or less, preferably, the parameter Z should be 12 or more as for<A>, and the temperature range should be 400 to 500° C. as for <B>.

In the invention, as described above, in consecutive multipass rolling,if the rolling temperature T_(x-out) right after x-th pass is higherthan the rolling set temperature Ts, rolling should be preferablystarted after waiting until the rolling entry temperature T_(x+1-in) ofx+1-th pass becomes T_(s)+20≧T_(x+1-in), or by preliminarily measuringthe processing heat generation T_(xH) at x-th pass, it may be desired toset rolling entry temperature T_(xs)≧T_(x-in)≧T_(xs)−T_(xH) supposingT_(xs) to be the rolling set temperature.

The temperature control may have temperature changes as mentioned above,or in continuous rolling, it may be designed to reheat in the midst ofrolling to compensate for material temperature drop, or to cool by forcein the mist of rolling for suppressing the temperature rise of thematerial.

In the invention, the temperature refers to the material surfacetemperature.

In the invention, the total cumulative reduction strain increases alongwith increase of strain due to formation of fine crystal particles fromthe processing particles flattened by warm process, and in order toobtain a structure almost completely composed of superfine crystals, atleast strain of 1.5 is needed, or more preferably strain of 2 or more.

In this case, the strain is a plastic strain, or reduction of areaconverted to true strain, and averages for rolling may include variousrolls, and in the case of bar steel, it may be rolled by groove roll.

In the invention, mechanism for enhancing the strength by phasetransformation is not used, and addition of alloy element is not neededfor enhancing the strength, and the steel composition is not limited,and various steel types free from phase transformation such as ferritesingle phase steel and austenite single phase steel, and steel materialsof wide composition range can be used. A more specific example ofcomposition comprises, by wt. %,

C, 0.001% or more to 1.2% or less,

Si: 0.1% or more to 2% or less,

Mn: 0.1% or more to 3% or less,

P: 0.2% or less,

S: 0.2% or less,

Al: 1.0% or less,

N, 0.02% or less,

Cr, Mo, Cr, Ni: 30% or less in total,

Nb, Ti, V: 0.5% or less in total,

B: 0.01% or less, and

balance of Fe and inevitable impurities, not containing any alloyelement. Alloy elements such as Cr, Mo, Cu, Ni, Nb, Ti, V, B may beadded more than the specified range as required, or may not be containedat all.

Embodiments are shown below and described further. It must be noted,however, that the invention is not limited to these examples alone.

EMBODIMENTS

Table 2 shows the composition (balance Fe) of steel materials in thefollowing examples.

In these examples, cooling is air cooling.

TABLE 2 (mass %) Type of steel C Si Mn P S N s.Al a 0.15 0.3 1.5 0.010.001 0.001 0.03 b 0.10 0.3 1.5 0.01 0.001 0.001 0.03 c 0.05 0.3 1.50.01 0.001 0.001 0.03

In Tables 3 to 6 below, the blank right column of pass No. refers toso-called “common passing,” that is, the same caliber is passed twice,and hence the reduction of area is indicated at the second pass.

The parameter Z is calculated in the final pass because it must becalculated after application of a specific strain. The symbols are asfollows: t=total time, T=exit average temperature, ε=total strain.

Embodiment 1

A steel material of 80×80×600 mm having the composition shown in Table2a was heated to 500° C., rolled in caliber at rolling set temperatureT1 (499° C.), and rolled in 21 passes at reduction of area of 91% (truestrain 2.4) until the section was reduced to 24×24 mm. Supposing thetotal rolling time of 600 s, the set value of Z was 15.0. From FIG. 1,the ferrite grain size is estimated to be 0.4 micron.

Caliber shape of each pass is shown in FIG. 2, and the draft andtemperature changes before and after are given in Table 3. Right afterrolling of even-number pass, the temperature (exit temperature)T_(x-out) was measured, and when the exit temperature was lower than500° C., rolling of next pass was started immediately, but whenexceeding 500° C., rolling of next pass (odd-number pass) was startedafter waiting until the material temperature dropped below 499° C. As aresult, the rolling was carried out in a range of entry temperatureT_(x-in) of 455 to 499° C. (average 495° C.) and exit temperatureT_(x-out) of 472 to 543° C. (average 520° C.). Therefore, although themaximum processing heat generation in rolling of two consecutive passeswas 40° C., by setting the rolling waiting time, the rolling settingtemperature of 499° C. did not exceed 550° C. throughout the wholerolling process. In other words, processing heat generation occurred ineach pass, but did not exceed the set temperature by more than 50° C. Bythe total rolling time of 895 s and average exit temperature, the valueof Z was calculated again, and Z=14.2 was obtained. The calculatedferrite grain size was 0.45 micron.

In the obtained bar steel, the C section microscopic image is shown inFIG. 3. The composition is ultrafine equiaxial ferrite+cementitetexture. The average ferrite grain size was 0.6 micron. Its mechanicalproperties are shown in Table 7, and a steel bar of excellent tensilestrength of 788 MPa was obtained.

TABLE 3 Reduction Total of reduction Total strain Pass interval Entrytemperature Exit temperature Pass No. area (%) of area (%) (—) (s)Rolling wait T_(x-in) (° C.) T_(x-out) (° C.) Caliber Z value 1 0 455472 #1 2 2.5 2.5 0.03 27 472 #1 3 5 495 #2 4 21.5 23.4 0.27 19 495 #2 57 530 #3 6 21.6 39.9 0.51 132 Wait 499 #3 7 8 533 #4 8 18.4 51.0 0.71 92Wait 499 #4 9 5 538 #5 10 20.3 60.9 0.94 115 Wait 499 #5 11 5 534 #6 1219.0 68.4 1.15 103 Wait 499 #6 13 17.0 73.7 1.34 5 528 #7 14 18.6 78.61.54 72 Wait 499 #8 15 15.6 81.9 1.71 10 543 #9 16 16.9 85.0 1.90 90Wait 499 #10 17 12.5 86.9 2.03 11 534 #11 18 6.8 87.8 2.10 88 Wait 496#12 19 13.8 89.4 2.25 9 517 #13 20 49 Wait 498 #14 21 14.8 91.0 2.41 7501 #14 14.2 22 38.0 123 Wait 450 514 #15 23 18.0 94.9 2.98 130 Wait 464537 #16 14.1

Embodiment 2

In succession to embodiment 1, further 2 passes were rolled until 17×17mm. Caliber shapes are oval and square. Both are large in deformation,and the processing heat generation was measured by preliminaryexperiment. As a result, it was found that the material temperaturerises by 80° C. by consecutive rolling of 2 passes. Accordingly, inpasses 22 and 23, the entry temperatures T_(22-in), T_(23-in) were setat 450° C. Since the temperature of pass 21 was 501° C., by waitinguntil the material temperature dropped to 450° C., and rolling of pass22 was started. The exit temperature of pass 23 was 514° C. At pass 23,waiting until 464° C., rolling was started, and the exit temperature was537° C. (Table 3). The total rolling time was 1112 s, and the Z valuewas 14.1.

The obtained texture image is shown in FIG. 4. An ultrafine equiaxialferrite+cementite texture was noted. The average ferrite grain size was0.5 micron. Its mechanical properties are shown in Table 7, and a steelbar of excellent tensile strength of 830 MPa was obtained.

Embodiment 3

A steel material of 80×80×600 mm having the composition shown in Table2b was heated to 900° C., and after once austenizing the texture, thematerial temperature was lowered to rolling set temperature T1 (550° C.)to transform the texture to ferrite+pearlite, and it was rolled incaliber in 20 passes at reduction of area of 91% (true strain 2.4) untilthe section was reduced to 24×24 mm. Supposing the total rolling time of600 s, the set value of Z was 13.7. From FIG. 1, the ferrite grain sizeis estimated to be 0.6 micron.

Caliber shape of each pass, the draft and temperature changes before andafter are given in Table 4. Right after rolling of even-number pass, thetemperature (exit temperature) T_(x-out) was measured, and when the exittemperature was lower than 550° C., rolling of next pass was startedimmediately, but when exceeding 550° C., rolling of next pass(odd-number pass) was started after waiting until the materialtemperature dropped below 570° C. The entry temperature of odd-numberpass was not particularly controlled. As a result, the rolling wascarried out in a range of entry temperature T_(x-in) of 440 to 557° C.(average 551° C.) and exit temperature T_(x-out) of 536 to 573° C.(average 551° C.). Therefore, although the maximum processing heatgeneration in rolling of two consecutive passes was 23° C., by settingthe rolling waiting time, the rolling setting temperature of 550° C. didnot exceed 600° C. throughout the whole rolling process. In other words,processing heat generation occurred in each pass, but did not exceed theset temperature by more than 50° C. By the total rolling time of 582 sand average exit temperature, the value of Z was calculated again, andZ=13.5 was obtained.

The obtained texture image is shown in FIG. 5. The composition isultrafine equiaxial ferrite+cementite texture. The average ferrite grainsize was 0.9 micron. Its mechanical properties are shown in Table 7, anda steel bar of excellent tensile strength of 702 MPa was obtained.

TABLE 4 Reduction Total of reduction Total strain Pass interval Entrytemperature Exit temperature Pass No. area (%) of area (%) (—) (s)Rolling wait T_(x-in) (° C.) T_(x-out) (° C.) Caliber Z value 1 2.5 2.50.03 0 550 #1 2 6 548 #2 3 21.5 23.4 0.27 27 545 #2 4 11 573 #3 5 21.639.9 0.51 66 Wait 550 #3 6 4 573 #4 7 18.4 51.0 0.71 68 Wait 557 #4 8 11571 #5 9 20.3 60.9 0.94 61 Wait 556 #5 10 17 573 #6 11 19.0 68.4 1.15 59Wait 557 #6 12 17.0 73.7 1.34 18 561 #7 13 18.6 78.6 1.54 56 Wait 552 #814 15.6 81.9 1.71 20 567 #9 15 16.9 85.0 1.90 25 Wait 553 #10 16 12.586.9 2.03 15 567 #11 17 6.8 87.8 2.10 57 Wait 550 #12 18 13.8 89.4 2.2525 545 #13 19 22 540 #14 20 14.8 91.0 2.41 14 536 #14 13.5 21 38.0 65Wait 500 568 #15 22 18.0 94.9 2.98 15 550 599 #16 13.5

Embodiment 4

In succession to embodiment 3, further 2 passes were rolled until 17×17mm. Caliber shapes are oval and square. Both are large in deformation,and the processing heat generation was measured by preliminaryexperiment. As a result, it was found that the material temperaturerises by 80° C. by consecutive rolling of 2 passes. Accordingly, inpasses 21 and 22, the entry temperatures T_(21-in), T_(22-in) were setat 500° C. Since the temperature of pass 20 was 536° C., by waitinguntil the material temperature dropped to 500° C., and rolling of pass21 was started. The exit temperature of pass 21 was 568° C. At pass 22,waiting until 550° C., rolling was started, and the exit temperature was599° C. The total rolling time was 662 s, the average exit temperaturewas 565° C., and Z was 13.6.

The obtained texture image is shown in FIG. 6. An ultrafine equiaxialferrite+cementite texture was noted. The average ferrite grain size was1.1 micron. Its mechanical properties are shown in Table 7, and a steelbar of excellent tensile strength of 712 MPa was obtained.

Embodiment 5

A steel material of 80×80×600 mm having the composition shown in Table2c was heated to 600° C., and rolled in caliber at rolling settemperature T1 (600° C.), in 21 passes at reduction of area of 95% (truestrain 3.0) until the section was reduced to 17×17 mm. Supposing thetotal rolling time of 300 s, the set value of Z was 13.1. From FIG. 1,the ferrite grain size is estimated to be 0.8 micron.

Right after rolling of even-number pass, the temperature (exittemperature) T_(x-out) was measured, and when the exit temperature waslower than 600° C., rolling of next pass was started immediately, butwhen exceeding 600° C., rolling of next pass (odd-number pass) wasstarted after waiting until the material temperature dropped below 600°C. The entry temperature of odd-number pass was not particularlycontrolled. As a result, the rolling was carried out in a range of entrytemperature T_(x-in) of 580 to 619° C. (average 590° C.) and exittemperature T_(x-out) of 610 to 648° C. (average 630° C.). Therefore,although the maximum processing heat generation in rolling of twoconsecutive passes was 40° C., by setting the rolling waiting time, therolling setting temperature of 600° C. did not exceed 650° C. throughoutthe whole rolling process. In other words, processing heat generationoccurred in each pass, but did not exceed the set temperature by morethan 50° C. By the total rolling time of 800 s and average exittemperature, the value of Z was calculated again, and Z=12.2 wasobtained. The obtained texture image is ultrafine equiaxialferrite+cementite texture. The average ferrite grain size was 1.4micron. Its mechanical properties are shown in Table 7, and a steel barof excellent tensile strength of 640 MPa was obtained.

Embodiment 6

A steel material of 80×80×600 mm having the composition shown in Table2a was heated to 500° C., and rolled in caliber at rolling settemperature T1 (475° C.), in 21 passes at reduction of area of 95% (truestrain 3.0) until the section was reduced to 17×17 mm. Right afterrolling of even-number pass, the temperature (exit temperature)T_(x-out) was measured, and when the exit temperature was lower than475° C., rolling of next pass was started immediately, but otherwise,rolling of next pass (odd-number pass) was started after waiting untilthe material temperature dropped below 475° C. so as not to exceed 500°C. (Table 5). The entry temperature of odd-number pass was notparticularly controlled. As a result, the rolling was carried out in arange of entry temperature T_(x-in) of 440 to 485° C. (average 465° C.)and exit temperature T_(x-out) of 472 to 499° C. (average 496° C.).Therefore, although processing heat generation occurs in each pass, butdoes not exceed the set temperature by more than 50° C. By the totalrolling time of 1128 s and average exit temperature, the value of Z wascalculated again, and Z=14.7 was obtained. The obtained texture image isultrafine equiaxial ferrite+cementite texture. The average ferrite grainsize was 0.45 micron. The tensile strength was 950 MPa.

TABLE 5 Reduction Total of reduction Total strain Pass interval Entrytemperature Exit temperature Pass No. area (%) of area (%) (—) (s)Rolling wait T_(x-in) (° C.) T_(x-out) (° C.) Caliber Z value 1 0 455472 #1 2 2.5 2.5 0.03 27 472 #1 3 5 495 #2 4 21.5 23.4 0.27 50 Wait 475#2 5 7 499 #3 6 21.6 39.9 0.51 130 Wait 475 #3 7 8 499 #4 8 18.4 51.00.71 90 Wait 470 #4 9 5 498 #5 10 20.3 60.9 0.94 116 Wait 475 #5 11 5498 #6 12 19.0 68.4 1.15 100 Wait 475 #6 13 17.0 73.7 1.34 5 499 #7 1418.6 78.6 1.54 70 Wait 475 #8 15 15.6 81.9 1.71 10 497 #9 16 16.9 85.01.90 91 Wait 470 #10 17 12.5 86.9 2.03 11 498 #11 18 6.8 87.8 2.10 85Wait 470 #12 19 13.8 89.4 2.25 9 495 #13 20 41 Wait 480 #14 21 14.8 91.02.41 7 499 #14 22 38.0 125 Wait 440 498 #15 23 18.0 94.9 2.98 131 Wait440 495 #16 14.7

Comparative Example 1

A steel material of 80×80×600 mm having the composition shown in Table 2was heated to 500° C., and rolled in caliber at rolling set temperatureT1 (550° C.), in 21 passes at reduction of area of 91% (true strain 2.4)until the section was reduced to 24×24 mm. The pass interval was 15 s.

Without any particular temperature control, results of rolling are shownin Table 6. The processing heat generation occurring in each pass wasaccumulated, and the final material temperature was more than 800° C. Atthe final exit temperature, the value of Z was calculated again, and Zwas 10.1. At the average temperature, it was 11.9. The obtained textureimage is shown in FIG. 7. Although it was a ferrite+cementite texture,the average ferrite grain size was 4 microns. The ferrite grain size waslarger than expected from the average temperature.

TABLE 6 Reduction Total of reduction Total strain Pass interval Entrytemperature Exit temperature Pass No. area (%) of area (%) (—) (s)Rolling wait T_(x-in) (° C.) T_(x-out) (° C.) Caliber Z value 1 0 550552 #1 2 2.5 2.5 0.03 10 549 #1 3 10 574 #2 4 21.5 23.4 0.27 10 571 #2 510 594 #3 6 21.6 39.9 0.51 10 591 #3 7 10 619 #4 8 18.4 51.0 0.71 10 616#4 9 10 639 #5 10 20.3 60.9 0.94 10 636 #5 11 10 664 #6 12 19.0 68.41.15 10 661 #6 13 17.0 73.7 1.34 10 684 #7 14 18.6 78.6 1.54 10 681 #815 15.6 81.9 1.71 10 709 #9 16 16.9 85.0 1.90 10 706 #10 17 12.5 86.92.03 10 729 #11 18 6.8 87.8 2.10 10 726 #12 19 13.8 89.4 2.25 10 754 #1320 10 751 #14 21 14.8 91.0 2.41 10 774 #14 22 38.0 10 771 814 #15 2318.0 94.9 2.98 10 809 839 #16 11.9

TABLE 7 Ferrite grain size Yield strength Tensile strength (μm) (MPa)(MPa) Embodiment 1 0.6 775 788 2 0.5 825 830 3 0.9 683 702 4 1.1 705 7125 1.4 600 640 6 0.45 940 950 Comparative example 1 3.1 480 560

INDUSTRIAL APPLICABILITY

As described specifically herein, the new method of controlling theparameter Z by the present invention can be applied to a continuousrolling process, and the invention provides a new control rollingmethod, in consideration of processing heat generation, as a method ofstably manufacturing ultrafine crystal steel of 3 microns to 1 micron orless, without any limitation in pass interval or strain speed.

1. A warm control rolling method, for manufacturing steel mainlycomposed of fine ferrite particle texture with average ferrite grainsize controlled by a desired value of 3 μm or less, which comprises,rolling the steel, in continuous multipass rolling at a rollingtemperature range of 350° C. to 800° C., in a condition where therolling condition parameter expressed in formula (1) $\begin{matrix}{Z = {\log\left\lbrack {\frac{ɛ}{t}{\exp\left( \frac{Q}{8.31\left( {T + 273} \right)} \right)}} \right\rbrack}} & (1)\end{matrix}$ ε: strain t: duration from start till end of rolling (s)T: rolling temperature (° C., or average of rolling temperature of eachpass in the case of multipass rolling) Q: 254,000 if mother phase oftexture just before rolling is ferrite, bainite, martensite, orpearlite; 300,000 if mother phase is austenite, is 11 or more (in thecase the texture just before rolling is ferrite, bainite, martensite, orpearlite, that is, Fe crystal structure is bcc) or 20 or more (in thecase the texture just before rolling is austenite, that is, Fe crystalstructure is fcc), and wherein at least one pass interval is longer than20 seconds, so that the material temperature upon start of rolling ofeach rolling pass does not exceed the maximum temperature of 800° C.,and the material temperature during rolling and right after finalrolling (within 1 second) is not 350° C. or lower, and so that, in eachrolling pass, temperature T_(x-out) right after rolling (within 1second) is not higher than temperature that is higher than rolling entrytemperature T_(x-in) by 100° C. and the material temperature right afterrolling (within 1 second) is not lower than temperature that is lowerthan the temperature right before rolling by 100° C.
 2. The warm controlrolling method of claim 1, wherein the temperature T_(x-out) right afterrolling in each rolling pass is not higher than temperature that ishigher than the rolling entry temperature T_(x-in) by 50° C.
 3. The warmcontrol rolling method of claim 2, wherein the rolling temperature rangeis 400° C. to 500° C.
 4. The warm control rolling method of claim 2,wherein Z is 12 or more and, the method is for manufacturing steelmainly composed of fine ferrite particle texture with an average ferritegrain size controlled by a desired value of 1 μm or less.
 5. The warmcontrol rolling method of claim 1, comprising rolling two or more passesconsecutively in rolling temperature range of 350° C. to 800° C.,wherein the material temperature right after two passes is not higherthan temperature that is higher than the material temperature upon startof rolling by 100° C., and not lower than temperature that is lower thanthe material temperature upon start of rolling by 100° C.
 6. The warmcontrol rolling method of claim 5, wherein the material temperatureright after two passes is not higher than temperature that is higherthan the material temperature upon start of rolling by 50° C.
 7. Thewarm control rolling method of claim 6, wherein the rolling temperaturerange is 400° C. to 500° C.
 8. The warm control rolling method of claim6, wherein Z is 12 or more and, the method is for manufacturing steelmainly composed of fine ferrite particle texture with an average ferritegrain size controlled by a desired value of 1 μm or less.
 9. The warmcontrol rolling method of claim 5, wherein the rolling temperature rangeis 400° C. to 500° C.
 10. The warm control rolling method of claim 5,wherein Z is 12 or more and, the method is for manufacturing steelmainly composed of fine ferrite particle texture with an average ferritegrain size controlled by a desired value of 1 μm or less.
 11. The warmcontrol rolling method of claim 1, wherein the rolling temperature rangeis 400° C. to 500° C.
 12. The warm control rolling method of claim 11,wherein Z is 12 or more and, the method is for manufacturing steelmainly composed of fine ferrite particle texture with an average ferritegrain size controlled by a desired value of 1 μm or less.
 13. The warmcontrol rolling method of claim 1, wherein Z is 12 or more and, and themethod is for manufacturing steel mainly composed of fine ferriteparticle texture with an average ferrite grain size controlled by adesired value of 1 μm or less.
 14. The warm control rolling method ofclaim 1, further comprising, in starting rolling in consecutivemultipass rolling, waiting until the rolling entry temperatureT_(x+1-in) of X+1-th pass becomes Ts+20≧T_(x+1-in) when the rollingtemperature T_(x-out) right after X-th pass is higher than chosenrolling temperature Ts.
 15. The warm control rolling method of claim 1,further comprising measuring the processing heat generation T_(xH) atX-th pass in multipass rolling beforehand, and defining the rollingentry temperature T_(x-in) in the relation ofT_(xs)≧T_(x-in)≧T_(xs)−T_(xH), where T_(xs) is a chosen rollingtemperature.
 16. The warm control rolling method of claim 1, wherein atotal reduction area in continuous rolling is 50% or more.
 17. The warmcontrol rolling method of claim 1, wherein a plastic strain, or a strainconverted into true strain from a reduction of area is 1.5 or more. 18.The warm control rolling method of claim 1, further comprisingintroducing the strain by multidirectional processing.
 19. The warmcontrol rolling method of claim 1, further comprising controlling thetemperature range before and after rolling by setting a rolling speedand a draft of each pass.
 20. The warm control rolling method of claim1, wherein the continuous rolling further comprises a step of reheatingin the midst of rolling for compensating for temperature drop ofmaterial, and a step of cooling in the midst of rolling for suppressingtemperature rise of material.